Respect DR 526 in implementation of std::[forward_]list remove/remove_if/unique.
[forward_]list::remove was already implementing it but the implementation has
been modified to generalize the following pattern. All nodes to remove are
collected in an intermediate [forward_]list which purpose is just to be
detroyed once out of scope.
libstdc++-v3/ChangeLog:
PR libstdc++/91620
* include/bits/forward_list.tcc (forward_list<>::remove): Collect nodes
to destroy in an intermediate forward_list.
(forward_list<>::remove_if, forward_list<>::unique): Likewise.
* include/bits/list.tcc (list<>::remove, list<>::unique): Likewise.
(list<>::remove_if): Likewise.
* include/debug/forward_list (forward_list<>::_M_erase_after): Remove.
(forward_list<>::erase_after): Adapt.
(forward_list<>::remove, forward_list<>::remove_if): Collect nodes to
destroy in an intermediate forward_list.
(forward_list<>::unique): Likewise.
* include/debug/list (list<>::remove, list<>::unique): Likewise.
(list<>::remove_if): Likewise.
* testsuite/23_containers/forward_list/operations/91620.cc: New test.
* testsuite/23_containers/list/operations/91620.cc: New test.
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